Add this code to your functions.php file in your theme editor to get rid of default fullscreen mode in WordPress 5.4+ :

if (is_admin()) {
function jba_disable_editor_fullscreen_by_default() {
$script = "jQuery( window ).load(function() { const isFullscreenMode = wp.data.select( 'core/edit-post' ).isFeatureActive( 'fullscreenMode' ); if ( isFullscreenMode ) { wp.data.dispatch( 'core/edit-post' ).toggleFeature( 'fullscreenMode' ); } });";
wp_add_inline_script( 'wp-blocks', $script );
}

add_action( 'enqueue_block_editor_assets', 'jba_disable_editor_fullscreen_by_default' );
}

Code credit: Jean-Baptiste Audras